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89’ 4.0 Xj rough idle/missing under acceleration

Jmedxj

NAXJA Forum User
Location
Paducah, ky
I just bought a 1989 cherokee laradoe 2dr 4.0 4x4 5sp , I noticed the idle when I test drove it but it was cheap enough so what the heck, so far I’ve checked the cps, tps, and fuel pressure is at 31/39 with line removed, ive pulled the plugs on the injectors while running and I can’t be sure 3 and 4 injectors make a difference, I’m not really sure where too head next. When I first start it cold it runs good for about 10-20 sec then the idle starts getting choppy almost like it’s cammed, the longer it idles the worse it gets it also surges like every 10secs or so it will Act like it’s gonna die then recover.
 
Hey cruiser thanks for the respond I found your site a couple days ago and have been going through your tips checking everything thanks for making that it’s invaluable. I checked the hose too the map and it looks good the p/o has replaced the injectors oxygen sensor fuel pump and pickup the fpr, egr valve, I’ve looked at all the routing and am certain it’s correct, the other day I went and drove it around for about 35 mins and came back home and shut her down and came back out too start it and it wouldn’t start waited 15mins or so and it started, it was like it quit firing wouldn’t hit at all. I got back too the shop unplugged the cps and pulled the fuel pump relay and coil wire, probed the cps and turned her over and i got 7.22AcV when cranking.
 
Okay so I bought a vacumme gauge and it’s reading 11-16 at idle depending on rpm 800-550 or so 20” at 2500rpm, it ran decent from a cold start today but gradually got worse as it warmed up. I shut it off after maybe 30mins changed the coil and tryed too restart it and it wouldn’t start again. Ughhhh... so I swapped old coil back.. still no start....
So I though we’ll im gonna test the cps again too see if I have a intermittent issue ohms out 281 cranking vac .728 again so idk wtf is going on.
I say the hell with it I’m gonna head home I stop for a minute looking at my phone and do a search with my symptoms and see a guy talking about his missing Cherokee and saying he sprayed all over his with carb cleaner and nothing happened and his boss came over with a can of brake cleaner and sprayed the manifold down by the back of the head, and it died. So I decided what the hell im gonna go try it I’m not a fan of wildly spraying a flammable substance on a hot exhaust/intake manifold but wth, so I spray a little and the a lot right at the back of the head and the mother ****er died! What does this mean???? I thought carb cleaner would make it rev up if it sucked it in? Someone help me make sense of this as it has never died before it started back up, I couldn’t replicate it again I ran out of fluid. Do I have a blown intake gasket? Could that cause a rough missing idle? And bucking missing on acceleration up too 3k then she takes off like crazy. PLEASE HELP!
 
Best guess is you have multiple issues going on. Sounds like most of it is the CPS.

Not starting hot is a typical CPS issue-281 ohms sounds high. Make sure and inspect the CPS wires and make certain they aren't touching the exhaust manifold.

That plastic tube for the MAP can get tiny cracks or rub spots usually on the bottom, I can feel them with my finger tips better than I can see them. Be real careful of that plastic tube, it gets brittle with age. Also make sure the MAP vacuum line is plugged into the proper hole, there are two, one goes all the way through and one is a blind plug.

Check the O2, knock sensor harness where it runs behind the power steering pump. If it was improperly routed during a water pump change it can be cooking on the exhaust.

When/if you get spark again, pull the number two plug wire, use an old plug opened to .060, hold it on a good ground and look at the spark, a fat yellow spark or a very weak looking spark isn't good, should be white blue and have an audible crack sound when it sparks.

Double check the torque on your exhaust manifold bolts, being real careful of the end studs, they get weak from heat and tend to rust solid and/or just crack, break and fall off. I skip the end studs unless I'm sure of them and just tighten the middles.

If you have a valve cover leak at the rear and it has oil soaked the CPS, it can/will cause issues.
 
Along with what has already been mentioned, with the low/fluctuating vacuum reading you may have the egr with a torn diaphragm or vacuum leak in the control circuit. When you get it running again and it starts it's surging thing warm, watch the stem of the egr, if it's moving in and out, lowering the idle when it pulls up and when it drops down raising the idle, this is the place to look. When mine went though, I had only a slight missing under acc so it does indeed sound like you have something else going on, especially with the thing choking out with the brake cleaner? For the intake/exhaust bolts, I remove the air cleaner box to get to the underside ones. I've also had I think #5 go between the in/ex on the factory gasket, granted, a long time ago but still, could be some of the issue.
 
Do a plug reading and post up the pix.
 
Good advice to check the EGR valve to possibly be stuck partially open. Sounds like you have a lean to rich cycles happening. This is result of unregulated air flow or fuel flow. I have not seen too many injectors leeking internally to cause this sort of issue...Not on theese jeeps anyhow. However have seen tons of air leaks. Check for a gross intake leak at the runners to the cly. head. Pretty common to suck in un-metered air there. The gasket is very sensitive to installation errors and is easily damaged. Spray with brake/carb cleaner with engine running while warm, if the leak is bad enough, it will want to flood and kill the engine when it is in a rich cycle. It will smooth out during a lean cycle for a very breif period before forcing a rich condition to it and you know what from there.
 
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